Nanotechnology-Enabled Sensors

3.6 Acoustic Wave Transducers 121
ronmental pollutants. 48 They are also employed for monitoring gas concentrations,
redox reactions occurring on metal oxide or polymeric sensitive
layers deposited on top of them. 49-51 QCMs operating in liquid media were
developed in 80s to measure viscosity and density. 52 They have also been
successfully realized as commercially available biosensing applications.
Further examples concerning these types of biosensors will be presented in
Chap. 7.
The efficiency with which a piezoelectric material can transform mechanical
waves to electromagnetic waves, and vice versa, is found from its
piezoelectric coupling coefficient, k 2 . In the past few decades, several crystals
have emerged that exhibit stronger piezoelectric effects than quartz,
These include: lithium niobate (LiNbO3), lithium tantalite (LiTaO3) and
recently lanagsite (LNG). LiNbO3 and LiTaO3 have k 2 s that are almost an
order of magnitude larger than that of quartz. These materials, however, do
have drawbacks, such as not being suitable for fabricating bulk type devices
due to their fragile structure and low quality factor (Q). However,
they are being rigorously investigated for surface acoustic wave applications.
3.6.2 Film Bulk Acoustic Wave Resonator (FBAR)
Film bulk acoustic wave resonators (FBARs) represent the next generation
of bulk acoustic wave resonators. They are comprised of thin films
and have much smaller dimensions than QCMs. They have a relatively
high operating frequency, which is the main reason for their higher mass
sensitivity. In addition, their fabrication is compatible with current Micro-
Electromechanical System (MEMS) standard technologies. 53,54 A typical
FBAR is built on a low-stress, inert membrane (such as silicon nitride)
with a piezoelectric film sandwiched between two metallic layers (Fig.
3.45).